LOPAC library screening identifies suramin as a TRIM21 binder with a unique binding mode revealed by crystal structure

Differential scanning fluorimetry screening of the Library of Pharmacologically Active Compounds (LOPAC) identified four hits for the PRYSPRY domain of the human E3 ligase tripartite motif-containing protein 21 (TRIM21). Isothermal titration calorimetry subsequently confirmed suramin as a binder with micromolar affinity. To further investigate the binding mechanism, mouse TRIM21 was used as a structural surrogate due to its improved protein stability and high sequence similarity to the human counterpart. A crystal structure of the complex refined at 1.3 Å resolution revealed a unique binding mode, providing new avenues for targeting TRIM21 and for the development of proteolysis-targeting chimeras (PROTACs).